In the field of mobile communication, successors to the third-generation mobile communication system are being discussed by a standardization group called 3GPP. For example, Long Term Evolution (LTE) is being discussed as a successor mobile communication system to W-CDMA, high speed downlink packet access (HSDPA), and high speed uplink packet access (HSUPA). A successor mobile communication system to LTE is also being discussed.
In such a mobile communication system, one or more resource blocks are allocated to user devices both in downlink and uplink communications. Resource blocks are shared by multiple user devices in the system. In each subframe of, for example, 1 ms, the base station performs a process called scheduling to select user devices to which resource blocks are to be allocated. A subframe may also be called a transmission time interval (TTI). In downlink, the base station transmits a shared channel using one or more resource blocks to user devices selected in the scheduling. This downlink shared channel may be called a physical downlink shared channel (PDSCH). In uplink, user devices selected in the scheduling transmit a shared channel to the base station using one or more resource blocks. This uplink shared channel may be called a physical uplink shared channel (PUSCH).
In a communication system where scheduling of radio resources is performed, it is necessary to signal (or report) allocation information for the shared channels to user devices basically for each subframe. As a downlink control channel for this signaling, a physical downlink control channel (PDCCH) or a downlink L1/L2 control channel may be used. The PDCCH, for example, includes the following information (see, for example, 3GPP R1-070103, Downlink L1/L2 Control Signaling Channel Structure: Coding, Jan. 15-19, 2007):                Downlink scheduling information        Uplink scheduling grant        Acknowledgement/negative-acknowledgement information (ACK/NACK)        Transmission power control command bit        
The downlink scheduling information may include information regarding a downlink shared channel. For example, the downlink scheduling information may include downlink resource block allocation information, identification information of user devices (UE IDs), the number of streams, information regarding precoding vectors, data sizes, modulation schemes, and information regarding hybrid automatic repeat request (HARQ).
The uplink scheduling grant, for example, includes information for an uplink shared channel such as uplink resource allocation information, identification information of user devices (UE IDs), data sizes, modulation schemes, uplink transmission power information, and information regarding a demodulation reference signal used in uplink MIMO.
The acknowledgement/negative-acknowledgement information (ACK/NACK) indicates whether retransmission is necessary for the PUSCH transmitted via uplink.
In uplink, the PUSCH is used to transmit user data, i.e., a normal data signal. Also, separately from the PUSCH, a physical uplink control channel (PUCCH) may be used to transmit, for example, downlink channel quality information (downlink channel quality indicator (CQI)) and acknowledgement/negative-acknowledgement information (ACK/NACK) for the PDSCH. The CQI is used, for example, for scheduling and adaptive modulation and coding (AMC) of the physical downlink shard channel. In uplink, a random access channel (RACH) and signals indicating requests for allocation of uplink and downlink radio resources may also be transmitted as necessary.
The acknowledgement/negative-acknowledgement information (ACK/NACK) can be basically represented by only one bit, but has the most essential role in retransmission control and greatly influences the system throughput. Therefore, the acknowledgement/negative-acknowledgement information (ACK/NACK) is preferably fed back as soon as it is generated. Also, as described above, the CQI indicates downlink channel conditions and is essential for scheduling and adaptive modulation and channel coding. Since the channel conditions constantly change, the CQI is preferably fed back to the base station at frequent intervals.
When resource blocks are allocated for uplink data transmission, the above control information can be promptly reported to the base station by using the resource blocks. However, there is a case where the CQI and the acknowledgement/negative-acknowledgement information for a downlink data channel need to be reported even when resource blocks are not allocated for uplink data transmission.
In the LTE system, single-carrier FDMA is used for uplink to make it possible to promptly report the CQI and the acknowledgement/negative-acknowledgement information to the base station even in such a case. Meanwhile, to further improve the frequency resource efficiency and the data rate, a multicarrier scheme is more preferable than a single-carrier scheme. However, technologies for efficiently transmitting the ACK/NACK and the CQI to the base station using a multicarrier scheme had not been fully discussed by the filing date of the present application.